You can upload both a Windows 8 and a Windows 8.1 package. Users on Windows 8.1 will get the Widows 8.1 version and users on Windows 8 will get the Windows 8 version.
If you provide only a Windows 8 version then Windows 8.1 users will download that. It should work the same as on Windows 8 and will not have access to the new features in Windows 8.1 (most obviously it will letterbox if the user sizes the Window to widths
between 320 and 768, which were not possible in Windows 8).
If you provide only a Windows 8.1 version then Windows 8 users will not be able to install it.
--Rob